Efficient collection and concentration of radiant energy is useful in a number of applications and is of particular value for apparatus that convert solar energy to electrical energy. Concentrator solar cells make it possible to obtain a significant amount of the sun's energy and concentrate that energy as heat or for generation of direct current from a photovoltaic receiver.
Large-scale light concentrators for obtaining solar energy typically include a set of opposed, curved mirrors, arranged as an optical system for concentrating light onto a receiver that is positioned at a focal point. As just a few examples, U.S. Pat. No. 5,979,438 entitled “Sunlight Collecting System” to Nakamura and U.S. Pat. No. 5,005,958 entitled “High Flux Solar Energy Transformation” to Winston et al. both describe large-scale solar energy systems using sets of opposed primary and secondary mirrors. As a more recent development for providing more compact collection apparatus, planar concentrators have been introduced, such as the solution described in the article entitled “Planar Concentrators Near the Etendue Limit” by Roland Winston and Jeffrey M. Gordon in Optics Letters, Vol. 30 no. 19, pp. 2617-2619. Planar concentrators employ primary and secondary curved mirrors, separated by a dielectric optical material, for providing high light flux concentration.
Some types of solar energy systems operate by converting light energy to heat. In various types of flat plate collectors and solar concentrators, concentrated sunlight heats a fluid traveling through the solar cell to high temperatures for electrical power generation. An alternative type of solar conversion mechanism, more adaptable for use in thin panels and more compact devices, uses photovoltaic (PV) materials to convert sunlight directly into electrical energy. Photovoltaic materials may be formed from various types of silicon and other semiconductor materials and are manufactured using semiconductor fabrication techniques. Photovoltaic components are provided by a number of manufacturers, such as Emcore Photovoltaics, Albuquerque, N. Mex., for example. While silicon is less expensive, higher performance photovoltaic materials are alloys made from elements such as aluminum, gallium, and indium, along with elements such as nitrogen and arsenic.
As is well known, sunlight contains broadly distributed spectral content, ranging from ultraviolet (UV), through visible, and infrared (IR) wavelengths, each wavelength having an associated energy level, typically expressed in terms of electron-volts (eV). Not surprisingly, due to differing band-gap characteristics between materials, the response of any one particular photovoltaic material depends upon the incident wavelength. Photons having an energy level below the band gap of a material slip through. For example, red light photons (nominally around 1.9 eV) are not absorbed by high band-gap semiconductors. Meanwhile, photons having an energy level higher than the band gap for a material are absorbed. For example, the energy from violet light photons (nominally around 3 eV) is wasted as heat in a low band-gap semiconductor.
One strategy for obtaining higher efficiencies from photovoltaic materials is to form a stacked photovoltaic cell, also sometimes termed a multijunction photovoltaic device. These devices are formed by stacking multiple photovoltaic cells on top of each other. With such a design, each successive photovoltaic cell in the stack, with respect to the incident light source, has a lower band-gap energy. In a simple stacked photovoltaic device, for example, an upper photovoltaic cell, consisting of gallium arsenide (GaAs), captures the higher energy of blue light. A second cell, of gallium antimonide (GaSb), converts the lower energy infrared light into electricity. One example of a stacked photovoltaic device is given in U.S. Pat. No. 6,835,888 entitled “Stacked Photovoltaic Device” to Sano et al.
While stacked photovoltaics can provide some measure of improvement in overall efficiency, these multilayered devices are costly to fabricate. There can also be restrictions on the types of materials that can be stacked together, making it doubtful that such an approach will prove economical for a broad range of applications. Another approach is to separate the light according to wavelength into two or more spectral portions, and concentrate each portion onto an appropriate photovoltaic device. With this approach, photovoltaic device fabrication is simpler, less costly, and a wider variety of semiconductors can be considered for use. This type of solution requires supporting optics for both separating light into suitable spectral components and concentrating each spectral component onto its corresponding photovoltaic surface.
One proposed solution for simultaneously separating and concentrating light at sufficient intensity is described in a paper entitled “New Cassegrainian PV Module using Dichroic Secondary and Multijunction Solar Cells” presented at an International Conference on Solar Concentration for the Generation of Electricity or Hydrogen in May, 2005 by L. Fraas, J. Avery, H. Huang, and E. Shifman. In the module described, a curved primary mirror collects light and directs this light toward a dichroic hyperbolic secondary mirror, near the focal plane of the primary mirror. IR light is concentrated at a first photovoltaic receiver near the focal point of the primary mirror. The secondary mirror redirects near-visible light to a second photovoltaic receiver positioned near a vertex of the primary mirror. In this way, each photovoltaic receiver obtains the light energy for which it is optimized, increasing the overall efficiency of the solar cell system.
While the approach shown in the Fraas et al. paper advantageously provides spectral separation and concentrates light using the same set of optical components, there are some significant limitations to the solution that it presents. As one problem, the apparatus described by Fraas et al. has a limited field of view of the sky because it has a high concentration in each axis due to its rotational symmetry. Another problem relates to the wide bandwidths of visible light provided to a single photovoltaic receiver. With many types of photovoltaic materials commonly used for visible tight, an appreciable amount of the light energy would still be wasted using such an approach.
Conventional approaches have provided only a limited number of solutions for achieving, at the same time, both spectral separation and high light flux concentration of each spectral component. These two goals are somewhat in conflict, and many conventional approaches to the problem of spectral separation would be difficult to implement in a compact optical system that must also provide high light flux concentration using a small number of components. Thus, it is recognized that there is a need for a photovoltaic cell that simultaneously provides both spectral separation and light concentration, that can be readily scaled for use in a thin panel design, that provides increased efficiency over conventional photovoltaic solutions, and that can operate with a substantial field of view in at least one axis along the traversal path of the sun's changing position across the sky.